Difference from average SST forecast for January to March 2026
| Photo Credit:
Australian Bureau of Meteorology
The La Nina weather phenomenon continues in the tropical Pacific, while the negative Indian Ocean Dipole (IOD) has ended, the Australian Bureau of Meteorology (BoM) said on Tuesday.
“The latest relative Niño3.4 SST index value for the week ending December 21, 2025, is −0.91 °C. Sustained values below –0.80 °C are consistent with a La Nina pattern. Weekly values of the relative Nino3.4 index have been fluctuating around the La Niña threshold since mid-to-late September,” BoM said in its latest climate driver update.
It said its model currently predicts that tropical Pacific Ocean temperatures are likely to remain at La Nina levels until early 2026 before returning to neutral, broadly consistent with most international models, but earlier than the typical ENSO event decay.
With the negative Indian Ocean Dipole (IOD) event ending, the IOD index was now back to neutral. “The Bureau’s model predicts the IOD is likely to remain neutral until at least the end of autumn 2026. This is consistent with all international models assessed. The IOD is typically inactive during December to April,” the Australian weather agency said.
Sea Surface Temperatures (SSTs) in the Australian region were the second warmest on record for November, with forecasts for December to February suggesting warmer-than-average SSTs are likely to persist across much of the region, particularly to Australia’s east. Elevated ocean temperatures can supply additional moisture and energy, intensifying the severity of storms, cyclones and rain systems.
Atmospheric indicators
Atmospheric indicators, including trade winds, pressure and cloud patterns over the equatorial central Pacific, have shown consistent signals of La Nina. As of 21 December 2025, the 30-day Southern Oscillation Index (SOI) has eased to −0.1, below the La Niña threshold of +7, reflecting recent tropical activity near Tahiti over the past few weeks.
The 60-day and 90-day SOI index values are +7.0 and +6.6, respectively, which are close to the La Nina threshold. Trade wind strength and cloud patterns have been indicative of La Niña conditions since at least mid-to-late September.
The Bureau’s model currently indicates that tropical Pacific Ocean temperatures are likely to stay at La Nina levels until early 2026, before shifting back to neutral. This is broadly in line with most international models, but earlier than the typical decay of an ENSO event.
The Southern Annular Mode (SAM) index is neutral as of December 20, 2025, and is forecast to remain neutral over the coming week. Beyond this period, there is a slight tendency towards negative values in the following fortnight. However, a range of possible outcomes remains, resulting in reduced clarity in the SAM phase after the coming week.
Globally, the pattern points to a climate system still carrying excess heat. Elevated ocean temperatures and a persistent La Nina are influencing rainfall, storm tracks and weather extremes across regions. While models indicate a shift back to neutral conditions in early 2026, the additional ocean warmth means weather impacts worldwide are likely to remain elevated in the months ahead, said BoM.
Published on December 24, 2025